This invention relates to a device for pressing a rack against a pinion as well as a rack-and-pinion steering system for motor vehicles with such pressing device.
Rack-and-pinion steering systems are known from the prior art in various configurations. Due to their operating principle, all rack-and-pinion steering systems include a steering gear with a rack and a pinion, wherein the pinion meshes with a toothed region of the rack. A rotational force applied onto the steering shaft and the pinion via the steering wheel is converted into a rack normal force and passed on to steerable wheels of a vehicle. Usually, rack-and-pinion steering systems nowadays are formed as hydraulic, electrohydraulic or electric power-assisted steering systems which support a vehicle operator in the steering operation.
Since considerable forces occasionally occur in the steering gear, it has already been recognized quite early that particular measures must be taken, in order to keep the rack in engagement with the pinion with as little backlash as possible. Otherwise, there is a risk that under load the rack moves away from the pinion by being deformed transversely to the longitudinal direction of the rack. There would at least occur an undesired increase of the backlash in the steering system, in the extreme case even slipping through of the steering system.
In order to prevent this, a thrust piece usually is employed in the region of the pinion, which urges the rack against the pinion with a rather constant pressing force. Adjusting the desired pressing force, taking account of wear phenomena as a result of the sliding friction between thrust piece and rack, which occurs during the steering operation, and avoiding disturbing rattling noise during the vehicle operation constitute the greatest challenges for pressing devices for rack-and-pinion steering systems.
U.S. Pat. No. 7,654,166 B2 already describes a pressing device for rack-and-pinion steering systems, which in operation of the vehicle operates largely free from backlash and hence particularly quietly and in addition allows an adjustment of the pressing force of the thrust piece.
To keep the pressing device largely free from backlash, two separate wedge members are provided in this document, whose inclined wedge surfaces engage the thrust piece. The two wedge members are urged against each other by a spring element transversely to a pressing axis, and via the wedge surfaces the radial force is converted into an axial force which axially presses the thrust piece against the rack. In this case, however, the assembly of the device, in particular the radial alignment and centering of the wedge members relative to the thrust piece, the provision of a sufficient axial pressing force by the radially acting spring element, and the exact pressurization of the thrust piece in axial direction via the two inclined wedge surfaces turns out to be expensive and problematic. An off-center or not exactly axially aligned pressurization of the thrust piece can lead to jamming of the pressing device and thus to an undesired “jerking” of the steering wheel during the steering operation.
DE 10 2010 039 202 A1 discloses a pressing device for rack-and-pinion steering systems, in which in the case of wear the thrust piece is to readjust itself by maintaining a desired backlash. For this purpose, the pressing device includes a housing part in which a piston loaded by a thrust piece spring in direction of the rack is shiftably guided. Furthermore a spindle is provided, which is coupled with the thrust piece by means of a first threaded connection not rotatable by axial force and is connected with a nut by means of a second threaded connection rotatable by axial force, wherein the piston forms a coupling with the nut and limits a movement of the nut relative to the piston in a direction facing away from the rack. The pressing device furthermore includes a readjusting spring which is arranged between the thrust piece and the piston and loads the thrust piece with a spring force in direction of the rack. Finally, the pressing device also comprises a stop which limits the path of the nut in direction of the rack. Due to its complex construction, however, the proposed pressing device is comparatively expensive in manufacture and assembly.